Mr. Duncan appears in Home Alone 2: Lost in New York.
He's a very kind man and he loves children. He gives money to children's hospitals. Kevin met him and when Kevin gave 20 dollars to the hospital, Mr. Duncan gives Kevin two turtledoves in return, and also advises Kevin to keep one of the turtledoves, and give the other to a very special person (as turtledoves are a symbol of friendship and love). Kevin then saw the picture of Mr. Duncan before he left, and he realizes that the kind man at the register was Mr. Duncan. Later, Kevin stopped Marv and Harry from robbing the toy store. Kevin wrote an apology letter to Mr. Duncan for breaking the store window while stopping the robbery, and thanked him for the turtle doves. Mr. Duncan tracked down Kevin and gave him and his family a truckload of free Christmas presents as a reward. Buzz and Mrs. McCallister found Mr. Duncan's name on the presents and they don't know who he is, and they didn't know Kevin knew him and saved his toy store.